A number of times on the show it has been mentioned that an exhaust system without back pressure can lead to burned valves, notably the exhaust valve. I’ve talked to a few friends with auto experience and they agree this is the situation. However, this seems counterintuitive, and none of us can fu…

Exhaust gas has very little oxygen in it compared to fresh air: ~1% vs 20%. It takes oxygen to burn metal. If a leaky or too-short exhaust pipe lets fresh air reach the exhaust valve it burns. The exhaust does not come out of the engine in a steady stream, it comes out in pulses. This is pretty …

Good explanation, now I get it.

First, most racing engines are rebuild frequently. In the case of drag racing, after several runs (less than 10) the motors of highly sponsored teams get rebuilt. Therefore some valve burning is expected. The valves and heads are specifically designed and treated to tolerate conditions that would…

I don’t necessarily agree that lack of back pressure will burn exhaust valves. If that were the case many an airplane would have fallen out of the sky due to short exhaust stubs. In fact, one of the methods of IDing an aircraft at night during wartime was to note the exhaust flame patterns. Runnin…

It does make sense in the ‘no exhaust manifold or pipes’ case. But once any length of pipe is in place, I agree, then I don’t see it.

It has appeared to me that an open exhaust first and foremost causes a significant loss of low and mid range torque but when the engine is operated at wide open throttle under load and suddenly the throttle is closed, cold air is gulped back and will often warp a valve. Warping leaves a gap at the s…

On street car you very rarely see burned valves anymore. I drove my Bronco for well over 150k miles with straight pipes, no cats, no mufflers, didn’t burn a valve or anything. My brother drove an F-150 with a similar exhaust for over 100k miles, and again no burned valves. It may have been more comm…

The valve warping explanation makes more sense than the first lines of my last post re. oxygen. Otherwise the Overrun Fuel Cutoff would be a problem. When the engine is above idle speed and the throttle is closed no fuel is injected or burned, so more oxygen goes through into the exhaust. At any…

I used to hear this theory about the burned valves and took it as truth without ever thinking i through. Now I’m not so sure. I think a lot might depend on other factors. Cars with open pipes seem to burn valves, but cars with open pipes also have other modifications intended to force more fuel int…

Open exhaust & burned valves

General Discussion

texases June 14, 2011, 1:39pm 2

I’ve heard that too, and even the more extreme claim, that running an engine without the exhaust manifold will cause complete valve failure (I think the mechanic at the gas station where I worked said something like ‘it’ll make the engine suck the valve’, whatever that means…). I don’t get it, either.

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Open exhaust & burned valves

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